(a) Field of the Invention
The present invention relates to a method and apparatus for determining a contention window size in a communication system. More particularly, the present invention relates to a method and apparatus for determining a contention window size in a communication system using a Listen Before Talk (LBT) method.
(b) Description of the Related Art
In a Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) method of a wireless Local Area Network (LAN), a busy status of a radio channel to transmit a packet is determined through sensing, and when it is detected that the radio channel is in a busy status by another wireless device, transmission is not started but stands by until a radio channel is in an idle status. If a radio channel is in an idle status, in order to avoid a collision, a determined transmission standby slot time is further stood by, and packet transmission is started. A transmission standby slot time is determined by a randomly selected backoff counter using uniform distribution at a predetermined contention window size.
When a response signal to a packet that is transmitted from the reception side is not received, a contention window size increases. A contention window size is binary exponentially determined, as in Equation 1, and such a method is referred to as Binary Exponential Backoff (BEB).Wi=W·2i  (Equation 1)where W is an initial value of a predetermined contention window size, and i is a backoff stage and is an integer sequentially having a value K representing a final (or maximum) contention window size state from 0 representing a minimum contention window size state. That is, by increasing one by one according to a predetermined condition such as a collision, i sequentially has values 0, 1, 2, 3, . . . , K.
A contention window size is first set to a minimum contention window size according to such a BEB method, and when a collision continuously occurs, a contention window size binary exponentially increases by Equation 1.
In IEEE 802.11 Distributed Coordination Function (DCF), a contention window interval according to a backoff stage i is determined to [0, CWi], and CWi is a value that subtracts 1 from Wi and has a relationship of Wi-1.
Because a conventional BEB method in which a contention window size exponentially increases, as in Equation 1, performs an important function in determination of a throughput and a short-term fairness performance of a wireless LAN, BEB performance enhancing methods have been suggested based on an increase speed of a contention window size up to now. Particularly, a recently newly suggested method includes a polynomial backoff method such as Equation 2.Wi=W·(1+i)x  (Equation 2)where x is an exponential value, and when x is 2, x is referred to as Quadratic Backoff (QB) and is represented with Equation 3.Wi=W·(1+i)2  (Equation 3)
It has been known that such a QB method is more excellent than an existing BEB method in a throughput and a short-term fairness performance of a wireless LAN.